Fiber lasers have demonstrated a great deal of potentials as high power pulse and cw laser sources, especially for applications where high quality, near diffraction-limited beam is required. Such applications include precision machining where well defined beam location is critical and micro-machining and waveguide-writing where a highly focused beam is a useful tool for reaching a threshold power level. The major limitation to the development of fiber lasers with even high peak power is nonlinear effects. The major nonlinear limits are from Raman scattering and self-phase modulation, although Brillouin scattering can also play a role in narrow line-width laser systems. Nonlinear coefficients are low for the silica glass used in most optical fibers. The interaction between the low nonlinear coefficients with high peak intensity in the small fiber core over a sufficient length can, however, still cause severe pulse distortion and loss of energy. Reduction of fiber length is certainly one possible approach. This is, however, limited by the solubility of rare earth ions in the glass host and M2 value of the multimode pump lasers. The key to the nonlinear problem is therefore optical fibers with large effective mode area while maintaining robust single mode propagation. Such fiber is also required to deliver a single mode beam over distance to the work piece, an important practical attribute in many applications.
Conventional single mode fiber can, in theory, be adapted to provide very large effective mode area. In practice, such a waveguide is so weak that the optical fiber becomes very sensitive to its environment, notably bending effects. Single mode propagation in fibers with few modes was subsequently proposed (see U.S. Pat. No. 5,818,630 for example). The robustness of the single mode propagation can be maintained at reasonable levels in this case especially when care is given to ensure single mode launch, minimization of mode coupling and additional mode filtering. A combination of these techniques has lead to a demonstration of single mode propagation with a mode field diameter (MFD) of ˜30 μm (A. Galvanauskas, “Mode-scalable fiber chirped pulse amplification systems”, IEEE J. Sel. Top. Quantum Electron., 7, 504 (2001)). Repeated efforts have also been made in the last few years to provide a large effective area solution using the emerging photonic crystal fiber technology. A typical photonic crystal fiber has a regular array of hexagonally placed air holes surrounding a solid core. A photonic crystal fiber supports guided modes in a solid core by proving a composite cladding comprising air holes in a glass background, having a lower effective refractive index than that of the core. To reduce the number of modes in photonic crystal fibers, the state-of-art design employs small air holes with hole-diameter d to pitch Λ ratio of less than 0.1. In this regime, the photonic crystal fiber is very weakly guided leading to high environmental sensitivity. Robust single mode propagation in photonic crystal fibers has been limited to a mode field distribution (MFD) of ˜28 μm (High-power air-clad large-mode-area photonic crystal fiber laser in Optics Express, vol. 11, pp.818-823, 2003), a similar level to that of conventional fiber. This is not surprising considering the similarity in principle of the two approaches. The progress towards fibers with large effective area is therefore relatively stagnant in the past 5-7 years, despite the significant progress in fiber lasers.